Low noise electron gun



Nov. 24, 1959 M. R. cuRRlE 2,914,699

Low NOISE ELEcTRoN GUN Filed Dec. 3, 1957 United Sttes Patent LOW NOISE ELECTRON GUN Malcolm R. Currie, Beverly Hills, Calif., assignor to Hughes Aircraft Company, Culver City, alif., a corporation of Delaware Application December 3, 1957, Serial No. 700,440

9 Claims. (Cl. 315-31) This invention relates to electron beam devices, and particularly to a low noise electron gun for such devices.

ln electron discharge devices, especially in amplifiers such as traveling-wave tubes, backward-wave tubes and klystrons, limitations on usefulness are frequently imposed by its own inherent noise generation which the tube may amplify to a level which is detrimental to the other functions of the tube. The conventional measure of this noise is designated noise gure and is delined as the signal to noise ratio at the input of the device divided by the signal-to-noise-ratio at the output. This is particularly true in traveling-wave tube devices, and especially true in backward-wave amplifiers. A voltage tunable backward-wave amplifier is frequently considered desirable for use in radar receivers. In such a utilization, maximum range of the radar system is determined by the noise figure of the tube; thus, it is advantageous to employ tubes having the lowest possible noise figure. Many other advantages of a low noise device will readily suggest themselves to those skilled in the art.

Low noise electron guns have heretofore usually consisted essentially of a diode region, that is, an emitting cathode and accelerating anode followed either by a series of accelerating anodes or a series of drift tubes operated at different potentials. In either case, the purpose of the region beyond the rst anode is to minimize and deamplify the noise in the beam. In the rst type, this is accomplished as gradually as possible by a change in potential as a function of distance from the cathode. In the latter type, it is accomplished by sudden velocity jumps which tend to minimize velocity fluctuations among the electrons in the stream. In both cases, the initial diode region is designed with considerable effort toward launching the beam in a uniform how. The resulting beam accordingly has a more or less uniform density across its width. It has previously been considered to be essential, in accordance with the theoretical models utilized, to launch the electrons as smoothly and evenly as possible from the emitting surface. The use of either a distorted potential prole or a nonuniform current density has been regarded as highly harmful to the noise figure of tubes, particularly those employing long electron beams. It has also been considered essential, according to the theoretical models used that a space charge limited type of emission be utilized in order to smooth out current uctuations in the beam near the cathode. In fact, all accepted prior art theoretical models have assumed a normal Fry-Langmuir potential distribution away from the cathode in order theoretically to minimize reduction in shot current and reduction in velocity fluctuations. "Il-lese models have lead to the prediction of a generally accepted minimum theoretical noise figure in the range of to 6 decibels which has been approached in practice and has been considered generally to be the best that could ever be attained. (See Proceedings of the I.R.E., vol. 43, pp. 981-991, August 1955, Minimium Noise Figure of Microwave Amplifiers, by H. A. Haus and F. N. H. Robinson.)

f. ICC

A prime object of the present invention is to provide an exceedingly low noise electron gun.

Another object is to provide a low noise electron gun for use in a beam type amplifier device having a noise figure considerably below what has been considered to be the theoretical It is another object to provide a low noise electron gun in which the improvement in noise reduction is achieved in the region between the cathode and anode.

It is another object to provide a low noise electron gun in which an unique arrangement of potentials and geometry of electrodes about the emitting cathode cause electron beam formation in a manner to obtain an exceedingly large amount of noise reduction.

Briely, these and other objects and advantages of the present invention are achieved in one embodiment by providing an arrangement including an emissive cathode which is immersed in a relatively strong magnetic iield which is coincident with the desired path of the electrons. The cathode has an emissive region lying within a short segment along the path. Disposed further along the path and spaced from the emissive region of the cathode is an accelerating electrode having a potential only slightly greater than that of the cathode, while disposed abeam of the emissive region of the cathode and extending both behind and in front thereof is a potential profile-shaping electrode having an inclined surface exposed toward the emitting region or segment and diver-ging away from the path at a predetermined angle. This profile-shaping electrode is maintained at a direct current potential which is more positive with respect to the cathode than is the accelerating electrode. This arrangement of electrodes and relative potentials produces a potential eld having a gradient near the edge of the emitting region or segment of the cathode which has a pronounced radial component which would normally tend to pull electrons toward the profile-shaping electrode. However, the strong axial magnetic field precludes such motion for electrons having the desired velocity, and the electrons are pulled toward the accelerating electrode because of the crossediield effects.

It is to be noted that the electrodes and potentials are arranged so that the electron current is pulled predominantly from that part of the emitting region Which is next to the edge of the cathode toward the proleshaping electrode. This applies to strip cathodes, annular cathodes and conventional planar disc cathodes.

The potential prole seen by these edge electrons in the absence of the rest of the beam is unique. It consists of a sharply distorted increase in potential awlay from the cathode followed by a relatively long region in which the potential is nearly constant or increases only very slowly as a `function of distance from the cathode. In this manner, a highly nonuniform current density across the cross-section of the beam is provided which discriminates against undesired velocities, reduces shot current and introduces a. statistical correlation between these noise quantities, that is, between shot current and velocity iiuctuation. This, then, qualitatively describes the theoretical model upon which the present invention is1 based.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, taken in conjunction with the accompanying drawing, in which like reference numerals refer to like parts, and in which:

Fig. 1 is a schematic view, partially in section, of an electron beam device constructed in accordance with the present invention;

Fig. Z'is a perspective view of a generalized embodi- Y 3 ment of the low noise electron gun of the present invention;

Fig. 3 is a view of a sheet beam-forrning electron gun embodiment; i Y

.Fig. 4 Vis a Viewl of-a-generalized-cylindrical embodiment of the electron'guno'the present invention;

Fig. 5 is V,a view, partiallyvin section, of amodied cylindrical vers'ionof the electron `gun of-thepresent invention; v Y y Fig. 6 is a sectional View of-a hollow cylindrical beamforming embodimentof the electron'gun of thepresent invention;

Fig.4 7 is -a ydiagram showing in -greater'ldetal the geometrical-relationships of certain 'features'lofp the electron gunof the presentinvention;

Fig. 8 is a pictorial graph plotting `relative current density across theemissive surface of a cathode of an electron gun of the present invention; V

Fig. 9` is a pictorial graph plotting relative electron current density across the emissivesurface Yota cathode of a hollow electron beam 'of the present invention; y

Fig. l is a graph plotting atypical `potential prole of an electron gun of the presentinventionin-which potential in volts is plotted as a function "of inches from the 'cathode edge along the axis of the tube.

Referring now to the drawings with 4more'particularity, Fig. V1 illustrates an embodiment ofthe present invention combining a low noise electron'gun with aparticular electron beam amplifying device, the particular example chosen being a traveling-wavetube amplilier 12.V A glass envelope 14 includes' an` enlarged diameter portion 16 at its lefthand extremity connected to an elongated Yportion of lesser diameter 18. Sealed to the' righthand extremity of the elongated portion '18 is a collector electrode 20. Disposed along substantially the ,length-of the elongated portion 18y is `a slow-wave structure 22 which is terminated on either end by arnat'ching ferrule 24. Coupled'to the input or lefthand end'of slow-wave structure 22 is a coaxial transmission line 26, the inner connector 28 thereof having a helical extension '30 which is wrapped'around'envelope 14 in a manner 'to Vprovide a contrawound helical tnansducer for launching 'traveling waves onto slow-wave structure 22,. Similarly, an output coaxial transmission line 32 is 'coupled to the output end ot slow-wave structure Q2 in a manner "to couple traveling wave signals off from slow-wave 22 andtransmit them to a utilization device, not shown.

Disposed within the enlarged portion of the envelope 1'4 is an electron gun 34 whichcomprises in this particular example a cylindrical cathode 36 `having a substantially planar emissive surface 38 at' its righthand end. Catho`de36 is heated in a conventional manner 'by a heating electrode 40. A profile-shaping electrode 42is disposed about the cathode 36in a 'mannerto substantially surround emissive surface 38 and extends both fore and aft ofthe emissive surface. The proiile-shaping electrode 42, along with other elements ofthe gun 34, maybe supported by .a set of dielectric rods 44 -which maintain axial `alignment and mechanical rigidity. Disposed further along the axis ofthe tube )l2V to the rightof the profile-shaping electrode 42 may be'placed a series of accelerating electrodes maintained usually at progressively more positive'potentials. Throughoutthis descripV tion only one or V.two such electrodes `are shown for pun poses of simplifying the discussion. The first accelerating electrode ld6, in Athis particular example, is an annular member having .an opening 48 of sufficient diameter to permit passage of an electron stream emitted by the cathode 36. Of similar geometry, and, spacedjurther along thetube 12, is. a second accelerating. electrode't) which has geometry similar to that of the electrode 46. .Both

accelerating electrodes are also'supportedqon 'the dielectric rods 44. The cathode 36 is maintained at a negative potential by a source of potential 52, the negativetermif nalof which may be connected to the cathode .736. A1; :an

appropriate tap, the heating electrode 40 is also connected to the source 52. The iirst accelerating electrode 46 is maintained at a positive potential with respect to the cathode by a connection to an appropriate tap, `as shown, and to the source 52. The profile-shaping electrode 42 is connected to the source 2 at a next more positive tap. The second accelerating electrode 5@ is maintained next more positive by an appropriate tap, also to the source 52.

Slowwave structure v22l may be maintained next more positivefas shown, by connecting it to ground potential through an` isolating resistor .54.

Surrounding substantially the entire length of the traveling-wave tube 12 is a focusing magnet 56, which may be a solenoid, as shown, and which is energizedby a source of potential 58. .The magnet 56 extends over the electron gun 34 and immerses it in a strong axial constraining magnetic eld so that electrons which would tend to be drawn toward the profile-shaping electrode 42 are instead constrained to flow in the axial direction along the length of the traveling-wave tube i2 withinthe slowwave structure 22. The electrons are eventually intercepted by the collector electrode V20 which dissipates their kinetic energy and conducts them to ground through Aa' source of potential 60. The source tlis connected with its positiveterminal to the collector 20 so that secondarily-emitted electrons will not drift backwardly into the interaction region of slow-wave structure 22 and cause added shot noise, or other deleterious effects.

Fig. 2 illustrates generally the geometrical relationships which are considered fundamental in an explanation of the structure of the electron gun ofthe present invention. The cathode V62 has an emitting surface 64 'flying within an-axially short segment 66 along the axialpredetermined path for an electron'stream 68. The .Cathode 62 is maintained at a reference .potential 'ofzero'volts A'proleshaping electrode surface 70^is disposed transversely'from the path'and extends axially "both behind and in front of the emissive segment66. The 'surface 70 is inclined in a manner `to be exposedtoward'the emissive segment 66 and diverges along the direction 'of the electron stream 68. T he vprofile-shaping velectrode surface i7() is maintained at a direct current potential V2 of a few volts positive respect'to the cathode'62 which is designated as being of potential V0 (as seen inFig..2) .of the cathode 62 and the profile-shaping electrodeV surface 76 Yis an accelerating electrode 72 which is placed substantially perpendicularly to the electron stream 68and .is closely adjacent thereto. The accelerating electrode 72'is main* tained at a vdirect current potential V1 which is intermedi'ateto' those of the cathode 62 and the prole-'shaping electrode surface 70. 'By'interrnediate is meant that the potential V1 is somewhere between the other two potentials.' The entire structure of Fig. 2 is immersed in a strong axial magnetic field which is parallelto the electron stream 68.. Y Y

Referring now to Fig. 3,`there is shown a sheet beamy forming electron gun in whichV a cathode 74 has an emissive surface 76 which is included within ,a short axial emissive segment 7S of the cathode'74. l A proleshaping electrode is disposed symmetrically vabove and below the emissive` segment 78 and has inclined proiileshaping surfaces 82 which are exposed toward the emissive segment 78 and diverge along the path'of theelectron stream 84. The surfaces 82 are contiguous tothe cathode 74 and extend both fore and aft of the emissive segment v 78. Disposed to the right of prole-shaping electrode 80 Fig. 3 is immersed in a strong axial magnetic field designated H.

Referring to Fig. 4, there is shown a cylindrical embodiment of the low noise electron gun of Ithe present invention. A cylindrical cathode 88 is axially aligned with the path of an electron stream 90 and is maintained at a direct current reference potential V0 which is equal to zero. The cathode 88 has an emissive surface 92 at its righthand extremity which is oriented substantially perpendicularly to the path of the stream 90. Disposed concentrically about the emissive surface 92 is a profileshaping electrode surface 94 having the form of a frustoconical surface of revolution, the determining planes of which lie respectively fore and aft of and parallel to the emitting surface 92. Disposed to the right along the path of electron stream 90 is an accelerating electrode 96 having an inner circular aperture which is concentric with`V the electron stream, and which is perpendicularly oriented with respect thereto. The cathode 88 is maintained at a direct current reference potential Vo of zero volts, the accelerating anode 96 is maintained at a potential V1 which is slightly positive with respect to the cathode, and the profile-shaping electrode 94 is maintained next more positive at a potential designated V2. The entire configuration is immersed in a strong axial magnetic field H.

Referring now to Fig. 5, there is shown a different one of a cylindrical beam gun including a cylindrical cathode 98 having an emissive end 100 lying within an axially short emissive segment 102. A profile-shaping electrode 104 is disposed perpendicularly to the axis of the cathode 98 and has a coaxial frusto-conical aperture 106 disposed about the cathode 98 and which extends both fore and aft of the emissive segment 102. The frusto-conical surface of aperture 106 is adjacent to the emissive segment 102 and diverges in the direction of the electron stream. The aperture 106 may comprise also surfaces 108 and 110 which are in this particular example cylindrical surfaces concentric Wit-h the axis of the cathode 98. Disposed to the right of the profile-shaping electrode 104 is an accelerating electrode 112 which has a circular aperture 114 concentric with and disposed contiguously to the electron stream emitted bythe cathode 98. The cathode 98 is maintained at a direct current potential V0 equal to zero volts, the profile-shaping electrode 104 is maintained at a direct current potential V2 which is la few volts positive with respect to the cathode, and the accelerating electrode 112 is maintained at a direct current potential V1 which is intermediate the potentials V0 and V2. The cathode 98, the profile-shaping electrode 104, and the accelerating electrode 112 are immersed in 'a strong axial constraining magnetic eld H.

Referring to Fig. 6, there is shown a low noise electron gun which produces a hollow annular electron stream 116. An annular cathode 118 'is maintained at a direct current reference potential V0 equal to zero Volts and is immersed in a strong axial constraining magnetic field H. A profile-shaping electrode 120 having a coplanar inner portion 122 is disposed perpendicularly to the axis of the cathode 118 and is axially oriented so as to extend both forwardly and rearwardly of the right hand emitting end 124 of the cathode 118. The profile-shaping electrode 120 has a circular aperture which comprises a frusto-conical surface of revolution 126 disposed concentrically about and radially contiguous to the cathode 118. The profile-shaping electrode portion 122 comprises an outer frusto-conical surface of revolution which is concentric with the cathode 11S and, as previously mentioned, is coplanar with proiileshaping electrode 120. The frus-to-conical surfaces of profile-shaping electrode 120 and profile-shaping electrode portion 122 each diverge along the direction of the electron stream 116 at the same angle of divergence. Disposed perpendicularly to the electron stream 116, to the right of the profile-shaping electrode 120, is an accelerating electrode 128 which has an annular aperture 130 concentric with and adjacent radially to the electron stream 116. Disposed further yet to the right (as seen in Fig. 6) is a second accelerating electrode 132 which is similarly oriented and of similar geometry with respect to the accelerating electrode 128. One or more additional accelerating electrodes may also be utilized here.

Referring now to Fig. 7, there is shown in more detail the dimensions and relationships of a low noise electron gun constructed in accordance with the invention. A cathode 134 is disposed along the axis of an electron stream 136 and has an emissive end portion 138 which is substantially planar and lies susbtantially perpendicularly to the axis of electron stream 136. The cathode 134 has a diameter or transverse dimension d and is maintained at a direct current potential V0 equal to zero volts. A profile shaping electrode 140 is disposed about and lies between Va transverse plane 142 which is aft of emitting portion 138 by a distance a, and a transverse plane 144 which is forward of the emitting portion 138 by a distance designated b. The surface of the proiile-shaping electrode 140 exposed toward the emissive end portion l13S is inclined at an angle /S from the axis of the cathode 134. The edge of the proiile-shaping electrode 140 determined by plane 142 is spaced from the cathode 134 by a distance designated e. Spaced forwardly from the plane 144 by a distance c is an accelerating electrode 146 which is disposed symmetrically above and below, or around, the electron stream 136 and 'is maintained at a direct current potential V1. The profile-shaping electrode 140 is maintained at a direct current reference potential of V2. Some typical ranges for the dimensions and voltages for optimum noise reduction ina practical embodiment, constructed substantially in accordance with the geometry of Fig. 7, are:

a-Between .005 inch and .200 inch; b-Between .005 inch and .060 inch; c-Between .020 inch and .100 inch; d-Between .005 inch and .200 inch; e-Between .003 inch and .070 inch; ,fi-Between 30 and 75 "g VQ-zero volts;

Vl-at or below V2.

These dimensions and voltage ranges are not given as being the limits for their respective parameters, but are merely given as empirical numbers which have proven to be especially successful in achieving noise reduction in electron guns constructed in accordance with the present invention.

Referring now to Fig. 8, there is shown a pictorial graph of relative electron current density on the horizontal scale versus position across the emitting surface 150 of a typical cathode 152, as is shown by the curve 148. The current is at a maximum near the edge of the cathode and decreases to a minimum. in either direction away from the edge.

Fig. 9 shows a plot similar to that of Fig. 8 in which a curve 154 plots relative electron current density horizontally versus radial position across the emissive surface 156 of an annular beam-forming cathode 158. The profile-shaping electrodes are not shown in either of Figs. 8 or 9, but the electron current density plots shown exist both by virtue of a profile-shaping electrode surface such as shown in the other figures which is symmetrically disposed about the emitting portions of the cathodes schematically shown in Figs. 8 and 9 and by virtue of the unique potential distribution. Again, in Fig. 9 it is seen that the electron current density is greatest near the edges of the cathode 158 and decreases to a minimum in either radial direction therefrom.

Referring to Fig. 10, there is shown the nature of the potential profile achieved by the various embodiments of the low noise electron gun of the invention. Plotted on the vertical axis is potential in volts versus thousandths of an vinch distance .from the emitting surface on .the The curve 160 represents the potential. which--would be seen by Vanrelectron emittedv near thehorizontal scale.

edge ofza cathode suchasthe cathode'134 of Fig. 7. The-curve 160 represents the potential seen by such an electron in the absence of space charge effects from the rest `of Vthe electron beam. The particularly .pertinent features to be noted. are the steep initial gradient represented by dotted line 162, which illustrates the gradient tending to initially accelerate Vedge-emitted electrons. 'Ihe long substantially gradient-free drift region between 10 and '76%` on curve loil represents the long drift region experienced by the emitted electrons.

A curve 164 represents the potential proiile experienced by an edgeemitted electron in the presence of the rest of the. electron beam. The profile of the curve 164 is sub- Stantially depressed from that of the curve 160 and represents that electrons inthe beam remain for a longer period of time .in a region of exceedingly low potential f at which the thermalrvelocity spread in the beam is comparable to the magnitude of the direct current potential,

In the operation of theelectron beam deviceof Fig. l microwave energy is delivered to the traveling-wavetube 12 along coaxial transmissionline 26 if the tube is being operated as a forward-wave device, or along coaxial transmission line 32, if the tube is being operated as a backward-wave amplifier device. In any event, microwave `energy lis propagated along the helix .22 and is caused to interact in a well-known manner ywith an electron beam projected from the electron gun 34 along the axis of the tube contiguous to the helix 22. The microwave energy in the form of a traveling wave having experienced interaction with the electron stream normally in a manner to achieve amplification, Iis then coupled oi of the helix 22 by the appropriate helical transducer and transmitted to a utilization device, not shown.

Referring to the operation of the electron gun 34, the cathode 36 is heated by current flowing through coil 40 causing thermal emission of electrons from the emissive area 38. The surrounding profile-shaping electrode 42 being maintained at a positive potential tends to cause electrons to tend to be pulled radially outward toward it. IHowever, the strong magnetic constraining eld maintained by the solenoid 56 causes the Yemitted` electrons to spiral and travel forward toward the slightly positive accelerating electrodes 46 and 50. 'It is to be noted that theinclined surface ofthe proleshaping electrodemaintainedat ahigher potential than the first yaccelerating electrode. as illustrated in each of Figs. l-7. is considered to be the preferred arrangement for providing the long drift profile of tFig. l0. Figs. y2 and 3 illustrate embodiments which provide ItheY desired potential prole along a strip, or rectangular beam. The inclined surface is clearly shown as being exposed toward the emissive portion of the cathode and diverging from the electron path. The strong axial magnetic field constrains the electrons to flow in the fonward direction in spite of the transverse-electric eld.

Figs. 4, 5 and 6 illustrate cylindrical beam forming embodiments in which again the orientation of the inclined surfaceofthe potential profile shaping electrode is clear. These figures represent embodiments which are particularly useful in traveling wave tubes such as backwardwave ampliers. In each of these iigures the inclined surface of the prole shaping electrode diverges from the path ofjthe beam and is maintained at a potential which would tend to pull the electrons outwardly from the edge ofthe cathode but for the strong constraining effect of the magnetic field H which forces them to spiral instead toward .the apertures ofthe accelerating electrodes.

The potentials utilized for the profile-shaping electrode 42 and the accelerating electrodes as well as their geometry are such as toprovidethelong drift region depicted by thegraph of Fig. l0. Hence, a number of factors affecting noise'reduction within the cathode-anode region comeinto play. Firstly,'there appears to be a considerable reduction in velocity fluctuations; secondly, a reduction in lshot current, electroncurrent fluctuations, is achieved; and thirdly, a strong statisticalzcorrelation between the remaining current density iluctuations versus .electronV rent density distribution across the Width ofthe electron.

bearn in order. to achieve noisereduction. infact, great elfort was expended in minimizing such distortions. lt is to be noted, in accordance with the theoreticalmodel here chosen, as previously discussed, that it is.considered that thesedistortions contribute to the noise reduction which has been found-to exist-in electron guns constructedin accordance with the present invention. YIt is to be noted further that prior artY techniques discussed above in the introduction, in accordance with the theoretical models there used have predicted a theoretical minimum noise figure of approximately 6 decibels, while. electron guns of the present invention which vdisregard .conventional theory Vand practice readily achieve noise figures of the order of 3 decibels.

Referring again to Fig. 10, although the curve has Vbeen measured and found to exist substantially as shown and in fact represents a solution of the Laplace equation, thecurve 164 which represents a practical solution of the Poisson equation is not so-well understood. It is substantially depressed due to spacecharge eects of the emitted electrons and presumably electrons emitted from near the edge of the cathode experience'the gradient indicated by the dotted line 162. Undoubtedly, the noise reductionis achieved within the relatively very long drift region between the numbers l0 and 80 on the abscissa. 'I'he usefulness of the Vpresent invention is not intended to be contingent on this explanation, the purpose of the explanation being merely to show some of the differences between the type of Vgun here utilized and the priory art geometrical and potential configurations.

=What is claimed is:

l. A low noise electron gun for projecting a stream of electrons in a selected direction along a predetermined path comprising: an emissive cathode; means for providing a constraining magnetic eld about said ,cathode in the selected direction along said path; a profile-shaping electrode contiguous to said cathode and disposed at right angles therefrom with respect to said direction and having a surface exposed toward said cathode and toward said direction at an exposure angle between said direction and said right angles; an accelerating electrode disposed about said stream and positioned along said path at a point farther from said cathode, said cathode being at a reference potential of zero volts, said accelerating electrode being at a potential positive with respect to said cathode, and said prole-shaping electrode being at a potential positive with respect to said accelerating electrode, whereby electrons are emitted from said cathode and are constrained by said magnetic field to ow in said direction Vthrough a substantially potential vfree region toward said accelerating electrode.

2. A low noise electron gun for providing an electron beam along a predetermined lineal path comprising: an emissive cathode; means for providing a magnetic ield along said path and about said cathode; a profile-shaping electrode disposed adjacent to said cathode at right angles therefrom with respect to said path, said profile-shaping electrode having a surface inclined toward said cathode and toward said direction, said surface forming an acute anglelwith said path, said cathode being at a reference potential of'zero volts, said accelerating electrode being -Adr at a potential positive withrespect to said cathode, said profile-shaping electrode being at a potential positive with respect to said accelerating electrode, lwhereby electrons are emitted from a portion of said cathode lying toward said profile-shaping electrode and are constrained by said magnetic eld to owin said direction through a substantially potential free region toward said accelerating electrode.

3. A low noise electron gun for producing a stream of electrons along a predetermined path comprising: an emissive cathode disposed at one end of said path; means for providing a magnetic field through' said cathode and along said path; an accelerating electrode disposed along said path and spaced from said cathode; a profile-shaping electrode disposed adjacent to and a beam from said cathode and having a surface inclined toward said cathode and said path at an angle with said path of 30 to 75 degrees and extending forward from said cathode by .005 to .060 inch, said cathode being at a reference potential of zero volts, said accelerating electrode being at a potential positive with respect to said cathode, said profileshaping electrode being at a potential positive with respect to said accelerating electrode, whereby electrons are emitted from said cathode and are constrained by said magnetic iield to ow in said direction through a substantially potential free region toward said accelerating electrode.

4. A low noise electron gun for projecting a stream of electrons along a predetermined path comprising: an emissive cathode having an emissive surface lying within a short axial segment along said path; a profile-shaping electrode disposed transversely to said path and having an axial thickness which is greater than the length of said axial segment and which extends both fore and aft of said axial segment of said path, said profile-shaping electrode having an inclined surface exposed toward said axial segment, said exposed surface forming au acute angle with said path; and an accelerating electrode disposed also transversely to said path and forward of said emissive axial segment of said cathode, said cathode being maintained at a direct current reference potential of zero, said accelerating electrode being maintained at a direct current potential which is positive with respect to said cathode and said proiile-shaping electrode being maintained at a direct current potential which is positive with respect to said accelerating electrode.

5. A low noise sheet-beam electron gun for projecting a stream of electrons along a predetermined path comprising: a rectangular emissive cathode having a planar surface disposed transversely to said path and included within a short axial segment thereof, at least a portion of said cathode within said axial segment being emissive; a prole-shaping electrode disposed parallel to said planar surface contiguously to said cathode and having an axial thickness which is greater than the axial length of said axial segment, said profile-shaping electrode having an inclined planar surface exposed toward said axial segment and toward the direction of said path; an accelerating electrode disposed substantially parallel to said profileshaping electrode contiguous to said path and spaced from said axial segment; and means for providing a relatively strong axial magnetic field throughout the region defined by the rear of said profile-shaping electrode and the front of said accelerating electrode, said cathode being maintained at a direct current reference potential of zero, said accelerating electrode being maintained at a direct current potential which is positive with respect thereto and said prole-shaping electrode being maintained at a direct current potential positive with respect to said accelerating electrode.

6. A low noise electron gun for projecting a stream of electrons along an axial path in a predetermined direction comprising: a cylindrical emissive cathode having a substantially planar end portion perpendicular to said axial path which is included within an axially short emitting segment of said cathode; a profile-shaping electrode disposed concentrically about said emissive segment and extending fore and aft thereof with respect to said path, said profile-shaping electrode comprising a frusto-conical internal surface of revolution exposed toward said emitting segment and diverging toward said direction; an accelerating electrode comprising a transverse sheet having an aperture concentric with said path disposed transversely to said path and spaced from said profile-shaping electrode; and means for providing an axial constraining magnetic field throughout the region defined by said proiile-shaping electrode and said accelerating electrode.

7. A low noise electron gun for providing a cylindrical beam of electrons along an axial path in a predetermined direction comprising: a cylindrical cathode member having an emissive end portion including a planar end and an emissive cylindrical surface adjacent thereto within an axially short emitting segment; a proiile-shaping electrode concentric with said cathode disposed between iirst and second planes which are transverse to said axial path, said first plane lying aft of said emitting segment and said second plane lying forward of said emitting segment and having a truste-conical internal surface of revolution disposed symmetrically about said axial path extending between said planes and diverging along said axial path; an accelerating electrode comprising a plane transverse to said path spaced from said profile-shaping electrode and having a circular aperture concentric about said axial path; and means for providing an axial constraining magnetic field extending throughout the region between said rst plane and said accelerating electrode, said cathode being maintained at a direct current reference potential of zero volts, said accelerating electrode being maintained at a direct current potential which is positive with respect thereto and said prolile-shaping electrode being maintained at a direct current potential which is positive with respect to said accelerating electrode.

8. A low noise electron gun for producing a hollow cylindrical stream of electrons along an axial path in a predetermined direction comprising: a profile-shaping electrode having a frusto-conical internal surface of revolution concentric about said axial path and diverging in said direction at a predetermined angle, the conical frustum being determined by rst and second planes perpendicular to said axial path, said profile-shaping electrode further comprising a second conical surface lying substantially between said first and second planes radially within said frusto-conical surface and converging at said predetermined angle in said predetermined direction, said frusto-conical and said conical surfaces being spaced from each other by a predetermined annular spacing; a cylindrical, annular cathode concentric about said axial path and having an axial short, emitting end segment, said cathode being disposed in the annular spacing between said frusto-conical surface and said conical surface such that said emitting end segment lies between said iirst and second planes; an accelerating electrode disposed in a plane perpendicular to said axial path and spaced axially from said profile-shaping electrode, said accelerating electrode having an annular opening concentric about said axial path, said annular opening being disposed contiguously to both the inner and the outer surfaces of said hollow electron beam; and means for providing an axial constraining magnetic eld throughout the region from said cathode to said accelerating electrode, said accelerating electrode being maintained at a direct current reference potential of zero volts, said cathode being negative with respect thereto and said profile-shaping electrode being positive with respect thereto whereby electrons are emitted from said emitting end segment of said cathode toward said profile-shaping electrode and are constrained by said magnetic field to oW along said axial path in said predetermined direction through a substantial potential free region to said accelerating electrode.

9. A low noise electron gun Ifor providing a cylindrical ing an emissive end portion including a planar end and an emissive cylindrical surface adjacent thereto Within an axially short emitting segment; a proleshaping electrode concentric with said cathode disposed between iirst and second planes lwhich are transverse to saidaxial path, said first plane lying aft of said emitting segment by .001 to .500finch, and saidv second plane lying forwardof said emitting segment by .001 to .100 inch-and having a truste-conical internal surface of revolution disposed symmetrically about said axial path atan angle of 20 to 80 degrees thereto extending between said planes ,having a minimum diameter at said rst plane and' a maximum diameter at said second plane thereby diverging lalong said axial path, the end of said frusto-conical surface having said minimum 'diameter being radially spaced from said cathode by a distance of from .001 to .100 inch; an accelerating electrode comprising a plane transverse to said path spaced from said prole-shaping electrode and having a circular aperture concentric and contiguous about said axial path; and means for providing an axial constraining magnetic iield extending throughout the region between said first vplane and said accelerating elec- ",tro`de,js`aid cathoderbeinglmaintainedat `a direct current reference potential f zero Vvf-voits," siaid'rac'celerating Velectrode "beingv maintaine'datfa :dirt/eci'.Y currentjpotential which is' .of the 'order 'ofthree yoltsfispositive With'respect theret'o "and said'proie-s'rlaping electrode jbeing rnaintainedY v at vadirect currentpotential whichis of theorder of six volts positive' withrespectV to `said Aaccelerating eiectrode.

i j V'Referer-ies'Sitedin the-fleofthispatent p UNITED .STATES 'PATENTS .Y

lFremlin- June 1, 1943 2,758,242 fSamueI Aug. `7, 1956 V2,792,518 "Quate f.. May 14, 1957 2,797,353 `Molnaret al. ;n June 25, 1957 2,800,602 Field Yet,a'l. July 23, 1957 2,806,972 'ViSenSiper' Sept 17, Y1957 v2,810,855)` ,Beek ;.a n' Oct.'22, 1957 2,813,222AV Diemer. 'Nov.y `12, 1957 2,817,035 'Birdsall.; Dec. 17, 1957 2,842,703 'Preist --.1 July 8, 1958 

